1. Field of Invention
The invention relates to apparatus and methodology for communicating on a local area network among a plurality of digital devices, and in particular to an improved bus protocol for use in such a local area network.
2. Description of Prior Art
The interconnection of computers and various input and output devices in a private or local area network is well known. In this manner, specialized input and output peripherals may be shared among a plurality of computer stations, such the sharing of as printers and memory archives. However, in any local area network communication between the computers and the peripheral devices must be coordinated and systematized, both for the ease and reliability of communication, and because a potential for conflicting demand on the peripherals exists. The architecture or means by which a plurality of computers on one hand and a plurality of input and output peripherals on the other hand are combined is ideally made user friendly for relatively unsophisticated system users to install and operate their own local area network, while at the same time providing the network, which is flexible enough to handle an expanded number of network users and peripherals.
One such prior art system is the systemizer manufactured by the assignee of the present application, Applied Creative Technology, Inc., of Dallas, Tex. In the Systemizer, an interface box is coupled to each computer user to be joined in the network to allow each network user to access any one of a plurality of printers which may be included in the network and to arbitrate conflicting demands upon those printers by other network users. In the case of conflict, a Systemizer allows the rejected network user to make an alternative printer selection. Computation and printer buffering is provided by the network to free up computer use while waiting for the selected printer to become available. Further, as a network arbiter with intelligence, the prior art Systemizer is capable of making multiple copies of specific documents, merging data from various sources into a single page, recovering and reprinting a document in the event of printer malfunction, translating and filtering data passing through it, providing remote control over a printer and adapting parallel and serial equipment.
However, as successful as the prior art Systemizer has been, it was incapable of bidirectional communication, namely from the output device back to the computer terminals of the network users. This feature is required if CAD plotters and modems are to used as input/output devices. Furthermore, if a printer was off line, the Systemizer has no means of detecting the status condition of the printer. Still further, if one of the peripherals was a modem for external communication from the network, the prior art Systemizer could not support modem operation by inclusion, for example, of any means for carrier detection. Intercommunication between the computers and the network was also difficult in the prior art Systemizer when used, for example, in electronic mail or file transfer.
Therefore, what is needed is an improved local area network apparatus and methodological protocol, which continues to provide each of the advantages and benefits of prior art devices, but is further enhanced and expanded to overcome each of the limitations or shortcomings of the prior art as discussed above.